Mechanism and Spread of Qnr-Mediated Resistance. Quinolones are widely used antimicrobial agents because of their broad antibacterial spectrum, low toxicity, and reliable action against otherwise resistant pathogens. Bacterial resistance to quinolones, however, is increasing and has reached alarming levels in some parts of Europe and the Far East. Various chromosomal mutations contribute to this resistance. Plasmid-mediated resistance was long thought not to exist. We discovered a plasmid-encoded protein termed Qnr that protects DNA gyrase from quinolone inhibition. Qnr acts additively with chromosomal mechanisms for quinolone resistance, belongs to the pentapeptide repeat family of proteins and, by a gel displacement assay, binds to the gyrase tetramer as well as to the GyrA and GyrB subunits with differing affinities. Although initially found at a single hospital in the United States, the qnr gene has recently been discovered in clinical Escherichia coli isolates from the Far East and in about 10% of quinolone resistant Klebsiella pneumoniae strains from the US. The aims of this proposal are to explore the hypothesis that Qnr blocks quinolone binding to gyrase, to study Qnr binding kinetics by surface plasmon resonance, to investigate whether Qnr can also protect gyrase from such protein inhibitors as MccB17, CcdB, and Gyrl, to study further the prevalence of qnr and the genetic basis of its acquisition by plasmids, and to explore whether Qnr and an active gyrase fragment can co-crystallize for structural analysis by x-ray diffraction. These studies are important not only for understanding an emerging resistance mechanism but should also reveal details of how DNA gyrase and related topoisomerases function and how proteins in the pentapeptide family interact with and regulate the activity of other proteins.